1. Field of the Invention
The present invention relates to a chip type directional coupler employing striplines.
2. Description of the Background Art
In order to manufacture a waveguide circuit, which has been a staple component of microwave circuits, highly precise machining is required. Therefore, such a waveguide circuit is unsuitable for mass production, and is high-priced, large-sized and heavy. In a radio set or a BS (broadcast satellite) receiver, therefore, microstrips or striplines are employed for implementing miniaturization and weight reduction through a high integration technique.
A directional coupler is a circuit element which is adapted to provide an output which is proportional to only unidirectional power from a source of microwave power flowing through a transmission line without reference to reverse power. FIG. 5 shows a conventional quarter-wavelength coupled-line directional coupler, which is formed by striplines 40 and 41. Referring to FIG. 5, microstripline electrodes 40a and 41a of the striplines 40 and 41 are partially close to each other horizontally over a length of .lambda./4, where .lambda. represents a wavelength.
Due to the coupling mode of the portions which are horizontally close to each other over the aforementioned length of .lambda./4, a fraction of the power which is applied to the principal line at a port 1 is produced at a port 3 of the secondary line.
Referring to FIG. 5, the stripline electrodes 40a and 41a are shielded by ground electrodes 42 and 43, shown with two-dot chain lines, which are arranged to enclose the stripline electrodes 40a and 41a on upper and lower sides thereof while being insulated from the stripline electrodes.
A function of such a directional coupler, which may be for halving a high frequency signal, for example, may be applied to a portable telephone, for example, for minimizing its transmission power. As shown in FIG. 6, a principal line 50a of such a directional coupler 50 is arranged between a transmission power amplifier 51 and an antenna 52 while an end of a secondary line 50b is connected to an automatic gain control circuit 53, to control the power of the transmission power amplifier 51 by means of the automatic gain control circuit 53.
However, it is important to further miniaturize the aforementioned portable telephone, and hence further miniaturization is required also for the directional coupler. As hereinabove described, each stripline electrode requires a length of .lambda./4, e.g., 7.5 cm at 1 GHz with a dielectric constant of 1. In order to couple linear stripline electrodes having such lengths, a substrate having a relatively wide area is required.